Modulation spectroscopy has become a very important experimental tool to study semiconductors (bulk or thin film), semiconductor structures (superlattices, quantum wells, heterojunctions), and semiconductor interfaces (Schottky barriers, metal-insulator-semiconductor, semiconductor-electrolyte, etc.). Modulation spectra in semiconductors range in amplitude from 10.sup.-6 to as much as 10.sup.-2 and are, therefore, relatively easy to measure with modern analog phase-sensitive detection as used in lock-in amplifiers for example, or digital data processing techniques. The derivative nature of modulation spectra suppresses uninteresting background effects and emphasizes structure localized in the energy region of interband transitions. In addition, weak features that may not have been seen in absolute spectra are greatly enhanced.
One of the most commonly used of the modulation spectroscopy techniques is that of measuring photoreflectance. The advantage of this technique lies in its contactless, nondestructive character and experimental simplicity. In fact it is a favored room-temperature optical technique for semiconductor microstructure characterization. Modulation spectroscopy measuring photoreflectance has typically been accomplished by utilizing a pump beam for the purpose of electromodulation of the sample. The main illumination (probe) beam is swept over the frequency range of interest while illuminating the sample for which measurement is desired. At the same time, electromodulation of the sample is produced by the photoexcitation of electron-hole pairs by the pump beam, which is typically modulated by a chopper circuit. The reflected beam is then focused onto a detector and the photoreflectance of the sample under excitation is measured. A detailed description of this prior art technique may be found in the article entitled "New Normalization Procedure for Modulation Spectroscopy" authored by H. Shen, P. Parayanthal, Y. F. Liu and Fred H. Pollak, appearing at page 1429 of the Review of Science Instrumentation, August 1987. This article is hereby made a part hereof.
Recently photoreflectance measurements have been performed at elevated temperature up to 690.degree. C. on GaAs, InP, AlGaAs, InGaAs, and AlGaAs/GaAs quantum wells. In spite of its success at high temperatures, there are only a few photoreflectance studies below 77 K. This is in part due to the fact that semiconductor samples (especially quantum wells and superlattices) tend to luminesce very efficiently. At low temperatures, the photoluminescence background is so strong that it masks the photoreflectance spectra. Two approaches have been used to solve this problem. One involves the use of a dye laser as a probe beam, and the other involves the use of a second synchronous monochromator before the detector as a band-pass filter.
The dye laser approach utilizes a laser that may be varied over a frequency range to generate the probe beam. This laser produces a more highly collimated beam of light than that produced by a lamp light channeled through a monochromator. Because this more highly collimated beam may be successfully detected if the detector is moved farther away from the sample the effect of the photoluminescence of the sample on the detector is reduced. The dye laser approach has the disadvantage of being more expensive than a lamp and monochromator system and of being more limited in frequency range than that system.
The use of a second synchronous monochromator for use as a bandpass filter before the detector stage will work, however some photoluminescence effect would still be present which would skew the reading. Further, synchronization of the monochromators has proven to be difficult.
The effects of these disadvantages and others are reduced or eliminated by the employment of applicants' invention as will be further described below.